A problem in connection with ordinary variable speed drives is that they either require high-resolution position measurement devices, or must utilize stepped current waveforms that increase motor heating and acoustical noise. These stepped current waveforms, referred to as trapezoidal waveforms or “6-step” waveforms when applied to a three-phase motor, can also produce significant torque ripple. The use of high-resolution position measurement devices requires that the position measurement device be properly aligned relative to the stator and rotor poles. Frequently this alignment is a manual operation, further increasing costs.
Algorithms for performing sensorless control of electric motors abound; but generally fall into two categories, based on how they determine the rotor position at start-up. The first type applies a large current to the motor so as to force the rotor to a known position, irregardless of its current position. This requires that the application be tolerant of backwards motion at start-up, and that the load on the motor at start-up is minimal. The second type relies on special features incorporated into the rotor so as to allow the rotor position to be determined by applying and measuring signals to the stator windings. This type of algorithm is not suited for the majority of electric motors on the market, as they lack the requisite rotor features.
It is well known to use phase-locked loops to produce an electrical signal whose frequency is a multiple of an input signal. In a variable-speed drive, this high-frequency signal can be used to estimate the rotor position given a low-resolution position input. For example, a multiply by 360 phase-lock loop can produce one degree resolution from a one pulse-per-revolution rotor position sensor. This approach has a critical limitation, however, in that it cannot be used at arbitrarily low speeds. In many applications, it is required to operate the motor at stall, and/or to reverse the motor direction. In such a case, the phase locked-loop cannot remain synchronized, as the rotor may come to a halt or reverse direction between sensor input transitions.
Thus, there is a need for a variable speed drive, one which allows for greater accuracy and control over variable speed motors when operating at speeds less than those optimal for accurate phase locked loop operation.